1. Field of the Invention
The present invention relates to a spindle servo circuit of a disc drive apparatus for use in an optical disc reproducing apparatus or the like.
2. Description of the Prior Art
Optical disc and magneto-optical disc recording and reproducing apparatus are known to optically read out recorded data from optical discs and magneto-optical discs. One of such optical discs is generally known as a compact disc (hereinafter simply referred to as a CD).
A spindle servo mechanism that rotates the CD, for example, is controlled to rotate the CD at a constant linear velocity. A linear velocity of the CD is determined in a range of from 1.2 to 1.4 m/s. A revolution rate of the 12-cm CD is about 500 r.p.m. in the inner periphery. This revolution rate, i.e., about 500 r.p.m. is changed to about 200 r.p.m. in the outer periphery of the 12-cm CD. Therefore, a loop gain of a spindle servo is changed in response to the change of revolution rate of the CD from its inner periphery to its outer periphery. Generally, in the 12-cm CD, a loop gain in the inner periphery is lower than that in the outer periphery by about 8 dB.
In addition to the 12-cm CDs, 8-cm CDs are now commercially available on the market. The 8-cm CD has a mass smaller than that of the 12-cm CD and therefore a moment inertia thereof is changed. It is generally known that a loop gain is changed with a difference of a moment of inertia. If the reproducing positions in the radial direction of the 12-cm CD and 8-cm CD are the same and the revolution rates at that reproducing position are the same in the 12-cm CD and 8-cm CD, then the difference of loop gain depends upon a mass ratio.
According to the standardization of CDs, the mass of 12-cm CD falls in a range of from 14 to 33 g and that of 8-cm CD falls in a range of from 6 to 16 g. A difference of loop gain with the same revolution number (=the same reproducing portion) fluctuations in a range of 1 to 14 (dB).
As is clear from the foregoing explanation, the servo gain is changed depending on the diameter of the disc to be rotated or a rotating frequency of the disc. U.S. Pat. No. 5,170,386 describes a method in which a spindle servo loop gain that changes in association with the change of linear velocity is constantly kept at constant loop gain regardless of the change of linear velocity. Japanese laid-open patent publication No. 4-245061 describes a method in which the type of disc is identified on the basis of a subcode recorded on a CD thereby to vary a servo loop gain.
Further, as a method of varying a servo loop gain, it is proposed to vary a gain of a driver circuit or to vary a frequency of a comparing signal and a reference signal of a servo control comparator. One well-known method of varying a gain of the driver circuit is to vary an input resistance or feedback resistance of an operational amplifier.
Furthermore, a loop gain of spindle servo in a CD signal processing IC (integrated circuit), which is manufactured by Sony Corporation under the trade name of CXD-2500, is varied as follows.
A signal that is recorded on a disc in advance is reproduced by an optical pickup. This reproduced signal is supplied to the above-mentioned signal processing IC, in which it is demodulated to provide an EFM (eight-to-fourteen modulation) signal. Then, a reproducing synchronizing signal included in the EFM signal at a predetermined interval is extracted by a reproducing synchronizing signal extracting circuit. The reproducing synchronizing signal thus extracted is frequency-divided by a frequency divider. The frequency-divided signal is supplied to a frequency comparator and a phase comparator as a comparing signal.
The frequency comparator and the phase comparator are supplied with a reference signal generated by a clock circuit formed of a crystal oscillator and compare a frequency difference and a phase difference between the reference signal and the comparing signal. Then, the frequency difference and the phase difference are measured by the counter and the gains of the frequency difference and the phase difference are varied by varying measured values.
As another means for varying a servo gain, there is known a method in which a servo gain is varied by varying a comparing signal frequency and a reference signal frequency of a servo control comparator. To be more concrete, a frequency-dividing ratio of a frequency divider is varied and also the reference signal is varied with the same ratio as the frequency-dividing ratio within a clock circuit, thereby varying the gain.
When the servo gain is varied by changing the gain of a driver circuit, a gain of an analog amplifier must be varied. There is then the disadvantage that a circuit arrangement thereof becomes complicated.
Furthermore, when the gain is varied by varying the comparing signal frequency and the reference signal frequency of the servo control comparator, a cut-off frequency of a low-pass filter that constructs a servo loop must be switched in a ganged relation to the change of the comparing frequency. As a result, a circuit arrangement for switching the comparing frequency becomes complicated.